1. Field of the Invention
This invention relates to a thermal head, and more particularly to a thermal head suitable for the facsimile.
2. Description of the Prior Art
A thermal head for the facsimile is usually constituted of an electrically insulating ceramic substrate, a glaze layer laid on the substrate, a tantalum-based or nichrome-based heating resistor formed on the glaze layer, and a plurality of first layer conductors provided at predetermined distances and in a predetermined shape on the heating resistor. The first layer conductor consists of two layers, for example, a chromium layer and an aluminum layer, formed by sputtering or electron beam vapor deposition. Usually, the chromium layer is formed on the glaze layer side.
A protective film is further formed on the exposed parts of the heating resistor, i.e. the parts having no first layer conductors on the surface of the heating resistor. The protective film is provided to improve the oxidation prevention and the wear resistance of the heating resistor, and usually is a film of two layers, i.e. a silicon dioxide (SiO.sub.2).sub.5 layer and a tantalum oxide (Ta.sub.2 O.sub.5).sub.6 layer. The silicon dioxide layer is often formed on the heating resistor side. The protective film is usually formed by sputtering or plasma CVD (chemical vapor deposition).
An interlayer insulating film made of polyimide resin is further formed on the first layer conductor, and throughholes are provided by photoetching the interlayer insulating layer. The interlayer is formed by coating the first layer conductor with polyimide resin and heating the coated first layer conductor at a temperature of about 350.degree. C., thereby baking the resin.
A second layer conductor consisting, for example, of laminates of a chromium layer, a copper layer and a gold layer is further formed on the interlayer insulating film and the throughholes by sputtering or electron beam vapor deposition. The thermal head is thus structured as above.
The thermal head as structured above has such a disadvantage that whiskers grow on the chromium layer and the aluminum layer of the first layer conductor, particularly on the aluminum layer due to the growth of aluminum crystal grains, depending on the heating history of the step for forming the interlayer insulating film made of the polyimide resin, and the growing whiskers break the interlayer insulating film to make a short circuit with the second layer conductor, i.e. to deteriorate the function of the thermal head.
A thermal head using inorganic silicon nitride (Si.sub.3 N.sub.4) as the interlayer insulating film in place of the organic polyimide is disclosed in Japanese Patent Application Kokai (Laid-open) No. 58-203068. However, it has been found that when silicon nitride is used as a material for the interlayer insulating film, cracks are formed on the interlayer insulating film during the formation of through-holes, and also that the interlayer insulating film is susceptible to a thermal shock during the printing, and once cracks are formed on the interlayer insulating film, the interlayer insulating film peels off at the locations of the cracks as the starting points owing to the shocks by the transfer of printing paper. It has been further found that, when silicon nitride is used as a material for the interlayer insulating film, the inside wall surfaces of throughholes as formed are vertically extended and when the second layer conductor is formed on the throughholes, the second layer conductor is discontinued at the vertically extended inside surfaces to deteriorate the connections.